Magic: The Gathering

It makes perfect sense, really. This sort of thing happens due to diffusion of responsibility in large enterprises.

The person making assurances to the public ("We don't use AI art") does not have control over whether those assurances are actually true. They themselves only have received assurances from someone else. They're passing a message along, and repeating it. Only when the public is skeptical does anyone within the organization actually go back and check carefully whether those assurances are really true.

What probably happened: Wizards contracted it out to a vendor who probably agreed not to use "AI art" in a boilerplate contract. That vendor hired a freelancer, and handed them the assignment. The freelancer used Photoshop inpainting features. The vendor returned the work to Wizards, who used it directly, trusting that the contract had been correctly fulfilled.